K. Pussi

The adsorption sites of rare gases on metallic surfaces: a review

During the past six years, the adsorption geometries of several rare gases in structures having several different symmetries on a variety of substrates were determined using low-energy electron diffraction (LEED). In most of these studies, a preference is found for the rare gas atoms to adsorb in the low-coordination sites. Only in the case of adsorption on graphite has a clear preference for a high-coordination site for a rare gas atom been found. This unexpected behaviour is not yet completely understood, although recent density functional theory (DFT) calculations for these and similar surfaces suggest that this is a general phenomenon. This paper reviews the early studies that were presages of the discovery of top site adsorption for rare gases, the discovery itself, and the present state of understanding of this curiosity. It also details some of the features of the LEED experiments and analysis that are specific to the case of rare gas adsorption.

A tensor LEED determination of the structure and compositional profile of a Cu{1 0 0}-c(2 2)-Pt surface alloy

Abstract The geometric structure and compositional profile of a Cu{1xa00xa00}-c(2×2)-Pt surface alloy formed by thermal activation of a monolayer Pt film has been determined by tensor low energy electron diffraction (LEED). A wide range of models have been tested. The favoured model consists of an ordered c(2×2) CuPt underlayer below a Cu terminated surface. Models involving a mixed ordered CuPt layer outermost may be definitively ruled out. The average T-matrix approximation (ATA) has been applied allowing variable Pt concentrations to be introduced into both the outermost layer and deeper into the selvedge (layers 3 and 4) in the form of a random substitutionally disordered Cu x Pt 1− x alloy. The favoured concentration profile corresponds to an almost pure outermost Cu monolayer ( Θ Pt =10±10 at.%) with Pt concentrations of 20±20 and 30±30 at.% in layers 3 and 4 respectively. Introduction of Pt into the surface layers induces a significant expansion of the selvedge yielding modification of the outermost three interlayer spacings to 1.84±0.02 A ( Δdz 12 =+1.9±1.1%), 1.91±0.03 A ( Δdz 23 =+5.8±1.7%) and 1.89±0.03 A ( Δdz 34 =+4.7±1.7%). The rippling in the first mixed CuPt monolayer is small and of amplitude 0.03±0.04 A with Pt rippled outwards towards the solid–vacuum interface.

A SATLEED study of the geometric structure of Cu{100}-Pd monolayer surface alloys

Abstract The structure of a Cu {1 0 0} -p(2×2) surface alloy formed by deposition of 1 ML of Pd on Cu {1 0 0} at room temperature has been studied by symmetrised automated tensor low energy electron diffraction. The favoured model from the wide range tested consists of a double layer ordered c(2×2) CuPd alloy with p(2×2)-p2gg symmetry introduced into the outermost layer via clock rotation of the CuPd monolayer with the corners of the p(2×2) unit cell centred over second layer Pd atoms ( R p =0.21). Lateral shifts of the top layer Cu and Pd atoms are determined to be 0.25±0.12 A. Substitution of 0.5 ML of Pd in both layers 1 and 2 leads to a significant expansion of the outermost two interlayer spacing to 1.93±0.02 A (+6.6±1.1%) and 1.90±0.03 A (+5.3±1.7%) and a rippling of Pd and Cu atoms in the outermost layer of 0.06±0.03 A with top layer Pd atoms rippled outwards. This model is in agreement with previous ion scattering studies of a Cu:Pd stoichiometry of 1:1 in the outermost two layers. A second mode of film growth consisting of adsorption of 0.5 ML of Pd on a copper capped Cu {1 0 0} -c(2×2)-Pd underlayer alloy leads to a structure which retains a simpler c(2×2) periodicity, suggesting that the growth of the p(2×2)-glide line phase requires a c(2×2) CuPd outermost template.

A tensor LEED study of an unusual cyclic hydrocarbon intermediate formed by benzene adsorption on Co(101̄0)

Abstract Tensor LEED has been used to determine the geometry of a p(3×1) phase formed by benzene adsorption to saturation coverage on Co (10 1 0) at 300 K. In contrast to benzene overlayers previously subjected to quantitative surface structural analysis, adsorption occurs with the ring in a tilted geometry across the substrate close packed [12 1 0] atomic rows in an off-centre bridge site with C s symmetry.

The ordering of a Xe monolayer on quasicrystalline Al–Ni–Co

The ordering of physically adsorbed gases on quasicrystalline surfaces exemplifies the effects of competing interactions. In this study, grand canonical Monte Carlo simulations were performed to complement experimental measurements of the ordering of Xe adsorbed on the tenfold surface of decagonal Al–Ni–Co. The simulations employed a semi-empirical gas-surface interaction, based on conventional combining rules, and the Lennard–Jones Xe–Xe interaction. The simulation results are consistent with the experiment and provide a new insight into the ordering behavior. The film initially has a fivefold quasicrystalline symmetry, but it evolves into a close-packed structure during adsorption of the second layer. The presence of symmetry defects in the sixfold structure creates domains of Xe having different (but equivalent) rotational epitaxy, suggesting that even in the absence of substrate defects, the annealed film has the five different rotational alignments observed in the experimental studies.

Determination of the structure of Cu{100}–c(4×4)-In by TLEED

Abstract Tensor low energy electron diffraction (LEED) has been used to study the structure of In adsorbed on Cu{1xa00xa00} surface at a coverage of 0.60±0.03 ML. At this coverage, a c(4×4) ordered surface structure is formed. The favoured structure is an overlayer in which the two top layers are pure indium. The indium layer closest to bulk has a c(2×2) periodicity, in which In atoms lie at fourfold hollow sites with respect to the substrate. This c(2×2)-In layer is overlayered by a c(4×4) layer in which indium atoms lie at fourfold hollow sites with respect to the c(2×2)-In layer. Indium atoms in the c(2×2)-In layer are laterally shifted off the hollow sites towards bridge sites by 0.28±0.09 A. Pendry reliability factor has been used to measure the level of agreement between theory and experiment giving a value of 0.28 for the favoured structure.